The present invention relates to an electrophotographic toner for developing an electrostatic image formed on a photoconductor surface in electrophotography, electrostatic recording or electrostatic printing, to a developer using the above toner, to a developing method using the toner, to a developing device using the toner, to a toner container containing the toner and to a developer container containing the developer.
Methods for visualize image information through electrostatic latent images by using an image forming device utilizing electrophotography or electrostatic recording are now utilized in a variety of fields. In the electrophotography, for example, image information is formed into an electrostatic latent image on a photoconductor through an exposing step following a charging step. The latent image is developed by a developer. Through succeeding transferring and fixing steps, the image information is reproduced. In this case, the developer may be a single component developer using a magnetic toner or a non-magnetic toner by itself or a two-component developer composed of a toner and a carrier.
The electrophotographic toner used for such a developer is generally produced by a kneading and grinding method in which a thermoplastic resin is melted and kneaded together with a pigment and, if necessary, a releasing agent and a charge controlling agent and, followed by fine pulverization and classification. The thus obtained toner is, if desired, added on surfaces thereof with inorganic or organic fine particles for the purpose of improving fluidity and cleaning property.
A toner obtainable by the conventional kneading and grinding method generally is irregular in shape and broad in particle size distribution and, thus, has problems that the fluidity is low, the transferability is low, the fixation energy required is high, the charge amount between particles is not uniform and the charging stability is low. Further, the quality of images obtained using such a toner is still unsatisfactory.
To overcome the problems of the above-described toner prepared by the kneading and grinding method, a method is proposed for preparing a toner by a polymerization method. Since this method does not include a grinding step, the toner can be produced without need of kneading and grinding steps. Therefore, this method contributes much to saving of energy consumption, reduction of process time, improvement in product yield and reduction of costs. Further, the particle size distribution of the polymerized toner particles obtained by the polymerization method is more easily adjusted to sharp distribution as compared with the grinding method. Additionally, the fluidity of the toner may be greatly improved and spherical toner may be easily obtained.
The toner produced by the polymerization method still has a number of problems to be solved. During the polymerization stage in the production of the toner by the polymerization method, surface tensions are exerted so that the sphericity of the particles is higher than that obtained by the kneading and grinding method. On the other hand, the characteristics of the toner are not fully satisfactory. Further, it is not easy to control the shape (irregularization of the shape) of the toner by the polymerization method. However, the toner is advantageous in charging stability and in transferability.
In a method of preparation of a toner by a suspension polymerization method which is widely adopted among various polymerization method, a monomer for the binder is predominantly a styrene monomer or an acrylic monomer which are harmful for human bodies. Since the toner obtained by this method inevitably contains these components, there is caused an environmental problem. Further, since the toner contains a wax, the deposition of the toner to a photoconductor during actual uses is reduced. However, since the wax is embedded in the toner, the wax is less easily exuded to the surfaces of the toner and, hence, the fixation efficiency is inferior as compared with the toner produced by the grinding method in which the wax is present on the surfaces of the particles. Therefore, the polymerized toner is disadvantageous with respect to the consumption of electric power. Further, when the amount of the wax of the polymerized toner used as a color toner is increased, or when the dispersion diameter of the wax is increased to improve the fixation efficiency, the transparency of the color image becomes worse so that the toner is ill-suited for use in the formation of images for presentation by OHP.
As a method of producing polymerized toner other than the suspension polymerization method, there is known an emulsion polymerization method which permits relatively easy control of the shape of the toner. In the emulsion polymerization too, the monomer is limited to a styrene monomer. Further, with this method, too, it is difficult to completely remove unreacted monomer components from the toner particles or to completely remove an emulsifying agent and a dispersing agent from the toner particles, so that an environmental problem by toner tends to occur.
A solution suspension method is known as a method of producing a toner. This method has a merit that it is possible to use a polyester resin which permits low temperature fixation. With this method, however, problems in productivity attributed to an increase of the viscosity of liquid are caused because a high molecular weight component is added in a step of dissolving or dispersing a resin with low temperature fixation property or a colorant in a solvent. In addition, for the purpose of improving cleaning property of the toner obtained by the solution suspension method, the toner is made spherical and the surface thereof is made uneven (Japanese Laid-Open Patent Publications No. H09-015903). Because of the irregular non-uniform shape, the toner is lacking in charge stability and has a problem in durability and releasability and, therefore, satisfactory toner quality is not obtainable.
Japanese Laid-Open Patent Publication No. H11-133665 discloses a dry toner having a practical sphericity of 0.90-1.00 and using a chain-extended, urethane-modified polyester as a toner binder for the purpose of improving toner fluidity, low temperature fixation property and anti-offset property. Japanese Laid-Open Patent Publications No. H11-149179 and 2000-292981 disclose a dry toner having excellent powder fluidity and transferability when formed into a small diameter toner and are also excellent in heat resistant preservability, low temperature fixing property and anti-hot offset property. The method of preparing the toner disclosed in these publications include a step of increasing molecular weight by condensation addition of an isocyanate group-containing polyester prepolymer with an amine in an aqueous medium.
The polymerized toner obtained by the above-described polymerization methods, the colorant is non-uniformly dispersed because of poor dispersibility thereof. Therefore, the image obtained by the toner has a problem that the transparency is low and the chroma (brightness) is inferior. In particular, there is caused a drawback that the image is dark when the toner is used to form a color image on an OHP sheet.
It is an objective problem of the present invention to provide an electrophotographic toner which uses a polyester resin as a binder, in which a pigment colorant is highly dispersed, which gives a high quality image excellent in transparency and chroma (brightness, gloss) and which exhibits excellent powder fluidity, anti-offset property, charge stability and transferability. The present invention is also aimed at the provision of a developer using the above toner, a developing method using the toner, a developing device using the toner, a toner container containing the toner and a developer container containing the developer.
The present inventors have made an earnest study with a view toward solving the above-described problems and, as a result, have completed the present invention.
Thus, in accordance with the present invention, there are provided a toner, a developer, a developing method, a developing device, a toner container and a developer container as follows:
(1) An electrophotographic toner obtained by dispersing an oil dispersion, containing an isocyanate group-containing polyester prepolymer dissolved in an organic solvent, a pigment colorant dispersed therein and a releasing agent dissolved or dispersed therein, in an aqueous medium in the presence of inorganic fine particles and/or polymer fine particles, reacting said prepolymer in said dispersion with a polyamine and/or a monoamine containing an active hydrogen-containing group to form an urea-modified polyester resin having an urea group, and by removing the liquid medium from the dispersion containing the urea-modified polyester resin, characterized in that said pigment colorant contained in the toner has a dispersion diameter, in terms of a number average diameter, of 0.5 xcexcm or less and in that particles of said pigment colorant having a diameter of 0.7 xcexcm or more account for 5% by number or less.
(2) A toner as recited in (1) above, characterized in that said pigment colorant has a dispersion diameter, in terms of a number average diameter, of 0.3 xcexcm or less and in that particles of said pigment colorant having a diameter of 0.5 xcexcm or more account for 10% by number or 10 less.
(3) A toner as recited in (1) or (2) above, characterized in that the toner has a weight average particle diameter of 3.0 to 7.0 xcexcm and such a particle diameter distribution that the ratio Dv/Dn (Dv: a volume average particle diameter, Dn: a number average particle diameter) is not smaller than 1.00 but not greater than 1.20.
(4) A toner as recited in any one of (1) to (3) above, characterized in that the toner has a sphericity of 0.900 to 0.960.
(5) A toner as recited in any one of (1) to (4) above, characterized in that tetrahydrofuran soluble components of the polyester resin contained in the toner has such a molecular weight distribution that a main peak is present in a molecular weight region of 2,500 to 10,000 and that the number average molecular weight thereof is in the range of 2,500 to 500,000.
(6) A toner as recited in any one of (1) to (5) above, characterized in that the polyester resin contained in the toner has a glass transition temperature of 40 to 65xc2x0 C. and an acid value of 1 to 30 mgKOH/g.
(7) A toner as recited in any one of (1) to (6) above, characterized in that said oil dispersion comprises a polyester resin dissolved therein and being non-reactive with said amine.
(8) A developer characterized in that the developer comprises a toner as recited in any one of (1) to (7) above, and a carrier.
(9) A toner container characterized in that the container comprises packed therein a toner as recited in any one of (1) to (7) above.
(10) A developer container characterized in that the container comprises packed therein a developer as recited in (8) above.
(11) A developing method characterized in that a toner as recited in any one of (1) to (7) above is used.
(12) A developing device characterized in that a toner as recited in any one of (1) to (7) above is used.
(13) A developing device characterized in that a toner contained in the toner container as recited in (9) above is used.
(14) A developing device characterized in that a developer contained in the developer container as recited in (10) above is used.
It is without saying that the toner according to the present invention is applicable as a monocolor toner and a color toner.